Increasing evidence suggests that abnormalities in synaptic transmission may be a key mechanism underlying some neuropsychiatric disorders. How might synaptic properties be specified an maintained? Synaptic cell adhesion molecules (SCAMs) are primary candidates that play a critical role regulating synapse function. Not surprisingly, genomic studies have found mutations in neurexin-3 (Nrx3) a prototypical presynaptic SCAM that is part of the neurexin/neuroligin complex. that are linked to Schizophrenia (SZ) and reward-seeking behavior. These disorders are associated with an enormous social and economic burden and share a common pathophysiological basis - dopamine system dysregulation due to abnormalities in synaptic transmission in the ventral subiculum (vSub) within the vSub-nucleus accumbens (NAc) shell circuit. Two distinct populations of subicular projection neurons serve as the major output of the hippocampus, receive input from CA1 and project to cortical and subcortical regions, including to two distinct medium spiny interneuron (MSN) cell- types in the NAc shell. A fundamental understanding how Nrx3 shapes cell-type specific synaptic properties at two subicular synapses involved in DA dysregulation - CA1 input to the vSub and subicular output to the NAc shell - is unexplored; thus, the dissection of cell-type specific pre- and post-synaptic functions of subicular neurons within this disease circuit may open new avenues for treatment strategies. To this end, we generated a Nrx3 mouse where splice site 4 (SS4), the splice site that dictates binding to nearly all neurexin ligands, is constitutively included (SS4+) but can be conditionally excluded (SS4-). We observed a selective reduction in AMPAR synaptic transmission, due to reduced surface AMPAR stability, in the dorsal subiculum of Nrx3SS4+ mice. Altered transsynaptic interactions in the Nrx3SS4 mouse may imitate disease-related Nrx3 mutations because most mutations affect surface exposed residues and alternative splicing - rarely resulting in complete protein loss. The overarching goal of this grant is to dissect presynaptic Nrx3SS4-dependent functions in distinct cell-types at the CA1-vSub synapse and the vSub-NAc shell synapse. The mentored aims will 1.) assess cell-type specific synaptic responses in vSub neurons that project to the NAc shell at the CA1-vSub synapse using stereotactic co-injection of retrograde virus and cre-recombinase in WT and Nrx3SS4 animals and 2.) assay vSub projections to two distinct populations of NAc shell MSNs in the striatal circuit by utilizing stereotactic injection of channelrhodopsin to selectively recruit ventral subicular fibers. For the R00 phase of this grant, will 1.) expand the study of synapse-specific striatal circuitry by dissecting the dorsal Sub-NAc core circuit using techniques acquired during the K99 phase and 2.) identify novel SCAMs by next-generation single-cell RNA sequencing of subicular neurons with the goal long-term of assessing their synaptic function in disease-relevant circuits. I anticipate that this proposal wil uncover new insights into cell-type specific synaptic properties in the subicular-striatal circuit and create a platform for future independent investigations into how other SCAMs function in subicular and striatal circuitry.